1. Field of the Invention
This invention relates to a method of extruding plastic or rubber compounds, and more specifically to a method in which the size and shape of the extrudate are controlled by varying extruder conditions.
2. Description of the Prior Art
An extrusion line comprises an extruder acting as a device for melting and mixing and as a pump to force molten plastic or rubber, or other material, through an irregularly shaped die. The extrudate produced by the extruder is then transported by a take-away device and is cooled in water or air while being transported.
In a cold-feed extrusion line, the output rate of the extruder is determined primarily by the speed of the screw inside the extruder. The screw speed governs the rate at which stock, always present in excess, can be advanced into the feed zone. In a hot-feed extrusion line, the average output rate of the extruder is determined by the average feed rate of hot stock which is supplied from a feed mill located some distance from the extruder.
While the extrusion line is designed to produce an extrudate having specified dimensions, deviations from the specified dimensions are continually being introduced by changes in material properties or by variations in the extrusion line operation. The dimensional variability can be divided into two categories: first, size deviations in which the cross-sectional area changes, but in which the ratios of the width and thickness dimensions (i.e., the shape) are constant; and, second, shape deviations in which the ratios of the width and thickness dimensions change, but in which the cross-sectional area or the weight per unit length (i.e., the size) is constant. In size deviations, all dimensions deviate from specification by the same percentage. In shape deviations, the dimensions deviate by different percentages from the specification, although the overall cross-sectional area remains the same.
In addition, there are two general categories of shape deviations which can occur. In the first type of shape deviations, all thickness dimensions change by approximately the same percentage, and all width dimensions change by approximately the same percentage, but the percentage changes for thickness and width dimensions are different and are of opposite sign. In other words, the extrudate thicknesses may all increase by a certain percentage, while the extrudate widths will all decrease by a different percentage. In the second type of shape deviations, different thickness dimensions change by different percentages, and the width dimensions remain essentially unchanged.
The control of size deviations by manipulating the speed of the screw in the extruder, or by manipulating the speed of the take-away device has been well known in the art. Examples of processes in which the take-away speed is manipulated to control size deviations are found in U.S. Pat. No. 4,087,499, issued to Bayonnet; U.S. Pat. No. 4,088,721, to Apicella; and U.S. Pat. No. 4,097,566 to Bertin et al.
Shape deviations, however, have been much more difficult to control by known means. One way of altering the shape has been to alter the geometry of the die by removing metal from the die opening or by narrowing the die opening by deforming the adjacent metal on the die face. This method has been difficult to use during a production run because it required production to be shut down during die modification and testing, and it very quickly caused a physical deterioration of the die.
Another approach for controlling shape deviations has been to use a die which could be adjusted by heated bolts or by a movable choker bar or mandrel or die sections. This approach has often been used for flat or annular dies. For typical production profile dies, however, this method requires a very complicated mechanical device. For tire tread extrusion, the adjustable sections of the die would also cause discontinuities along the tread profile.